The present invention relates to an air/fuel ratio sensor for use with an internal combustion engine and, more particularly, to an air/fuel ratio sensor capable of detecting deterioration of the sensing elements that may occur when the engine is operated without a fuel supply.
A conventional air/fuel (A/F) ratio sensor for use with an internal combustion engine employs two sensing elements each consisting of an oxygen-ion-conductive solid electrolyte having a porous electrode formed on both sides. This sensor has the ability to detect the concentration of oxygen in the exhaust gas from the internal combustion engine and produce an A/F signal.
In a typical sensor of this type, the two sensing elements are disposed in a face-to-face relationship with a small gap therebetween so as to provide a closed compartment that accepts a limited inflow of the exhaust gas. One of the two sensing elements is used as an oxygen concentration electrochemical cell while the other unit works as an oxygen pump. The pump current flowing through the oxygen pump is controlled such that the electromotive force generated by the electrochemical cell is maintained at a predetermined level and the resultant pump current is detected as an A/F ratio signal.
Air/fuel sensors may also be used with modern internal combustion engines to perform electronic feedback control over the fuel injection (A/F ratio). One type of electronic control performed on these engines is fuel cutting control in which the fuel supply to the engine is stopped when the vehicle is slowing down or running under low-load conditions.
The above-described conventional A/F ratio sensor designed to control the pump current and produce an associated A/F ratio signal may be employed to perform A/F ratio control on an internal combustion engine adapted for fuel cutting control. When a vehicle having such an engine and running at full throttle slows down, fuel cutting control is started to stop fuel injection. Since no fuel is injected into the engine, there is no need to perform control of the A/F ratio. But, in fact, the A/F ratio sensor will continue to perform control of the pump current, and hence the concentration of atmospheric air is detected as an associated A/F ratio signal.
The conventional A/F ratio sensor which will control the pump current and produce an associated A/F ratio signal is required to have a sufficiently accurate response that it is capable of precise A/F ratio detection in the vicinity of the stoichiometric A/F ratio where the exhaust gas has a comparatively low oxygen level. To this end, the A/F ratio sensor is provided with the capability of controlling the pump current flowing through the oxygen pump so as to produce a great difference between the oxygen partial pressure in the exhaust and that in the closed compartment in order to ensure that the oxygen concentration electrochemical cell will produce a predetermined high output voltage. Because of this design, if atmospheric air flows into the exhaust pipe when the car is running with no fuel supply, a large current will flow into the oxygen pump so that a large volume of oxygen is pumped out of the closed compartment. This, however, accelerates the deterioration of the oxygen pump and increases the change of failure in providing accurate A/F ratio detection.
In order to avoid this problem, a control circuit may be configured such that the maximum pump current flowing when the car is operating without fuel supply is within allowable limits. In this case, however, the sensitivity or response of the sensor will decrease with the decreasing concentration of oxygen in the exhaust, thereby making it difficult to accomplish accurate A/F ratio detection in the vicinity of the stoichiometric value.
In an A/F ratio detector, the sensing elements deteriorate as a result of prolonged exposure to the hot exhaust gas. In addition, dust particles may be deposited on the sensing elements for some reason. In either case, the affected sensing elements will produce an incorrect A/F ratio signal that cannot be utilized for the purpose of accurately controlling the A/F ratio of the exhaust from the internal combustion engine.